Most polychlorinated biphenyls (PCBs) are readily absorbed from the intestines of higher animals, but metabolism to more polar compounds is a prerequisite to PCB excretion. The retention of PCBs is inversely proportional to the rate of metabolism. It is the position and not necessarily the degree of chlorination that determines the rate of PCB metabolism. Two adjacent, unsubstituted carbon atoms are required for a rapid rate of PCB metabolism because their presence facilitates oxidative metabolism, probably via an arene oxide intermediate. However, PCBs without two adjacent unsubstituted carbon atoms may be metabolized via an arene oxide intermediate by shifting a chlorine, using the NIH shift, or by directly inserting a hydroxyl group into the meta position. The major metabolites of PCBs are mono- and di-hydroxylated derivatives which are excreted as glucuronide conjugates.